1. Field of the Invention
The present invention generally relates to information transmission systems having variable-length packets that carry prioritized data and, more particularly, to a diversity error control coding method and apparatus for use with such systems.
2. Background of the Invention
Some information transmission systems employ both variable-length packets as well as prioritized data (i.e., some data can be classified as low-priority and some can be classified as high priority). Examples of such systems include those having recent audio and video coding schemes.
Prior systems have included separating high and low priority traffic and differentially protecting them with different error control codes. Interleaving is also commonly used to spread/despread a burst of errors to make the block coding mechanism more efficient.
Other information transmission systems only employ variable-length packets and not prioritized data. In such systems, the prior art (see, e.g., J. Boyce, “Packet Loss Resilient Transmission of MPEG Video Over The Internet”, Signal Processing: Image Communication 15, pp. 7–24, 1999) discusses techniques to group several variable-length packets together and to form code words along the vertical dimension. FIG. 1 is a block diagram illustrating a grouping of variable-length packets 102A–F, according to the prior art. Code words are formed across several packets vertically, and the associated parity symbols are transmitted in a separate packet. For example, the first codeword is formed from the first symbol from each packet. The second codeword is formed from the second symbol from each packet, and so on. The associated parity bytes are transmitted in a separate packet. Since some code words are shorter than others, the size of the vertical code words will vary. A mother code can be used and variable-length codewords can be formed by shortening the mother code to the appropriate length.
The problem with the prior art techniques is that no additional protection is given to high priority data. One possibility is to group the high priority packets together and send them separately using more parity symbols. However, the drawback of this approach is that there will be increased latency while waiting for enough high priority packets. In practice, the high priority packets are arranged sparsely. For example, in video, a high priority packet might contain an I-frame and it will be in the middle of several predictive frames that would be marked as low-priority.
Accordingly, it would be desirable and highly advantageous to have a diversity error control coding method and apparatus for use with information transmission systems having variable-length packets that carry prioritized data that overcomes the above-described deficiencies of the prior art.